Table columns: dropdown/text filters per column

- tablekit builds a filter row under the headers for every th with data-filter;
  distinct cell values become a select (label from data-filter-label), or a
  text substring filter with data-filter="text"
- filters combine with the global table search; placeholder rows (colspan) are
  not filtered away; filter row follows the lab-focus column hiding
- enabled on: matrix lab columns (by variant), defense table (ЛР, status with
  readable values), students (Основа, Пароль), lab assignment lists (Уровень,
  Источник, Сдача), labs list (Статус)

web/src/main/resources/static/css/app.css

@@ -338,3 +338,10 @@ th.th-sortable.is-desc::after { content: " ▼"; color: var(--c-text-muted); fon
 .row-hidden { display: none !important; }
 .table-search { width: 14rem; }
 @media (max-width: 640px) { .table-search { width: 100%; } }
+.filter-row th { padding: 0.25rem 0.35rem; background: var(--c-bg); }
+.filter-control {
+    width: 100%; min-width: 3.5rem; font-size: 0.72rem; line-height: 1.4;
+    padding: 0.1rem 0.25rem; border: 1px solid var(--c-border); border-radius: 0.35rem;
+    background: var(--c-surface); color: var(--c-text); font-family: inherit;
+}
+th.th-filterable { padding-bottom: 0.35rem; }